1. Field of the Invention
The present invention relates generally to systems for cementing casing in a wellbore. In a further aspect, this invention relates to a system for reducing formation fracturing when cementing casing in an air drilled wellbore.
2. Discussion of Prior Art
During the construction of oil and gas wells a borehole is drilled to a certain depth. The drill string is then removed and casing is inserted into the borehole. After insertion of the casing into the borehole, cement slurry is pumped down through the casing and up into the space, or annulus, between the outside of the casing and the wall of the borehole. The cement slurry, upon setting, stabilizes the casing in the wellbore, prevents fluid exchange between or among formation layers through which the wellbore passes, and prevents gas from rising up the wellbore.
Casing which is lowered into the borehole is typically equipped with a check valve mounted on or adjacent to the bottom of the casing. The check valve is incorporated into a device commonly known as either a float collar or a float shoe. If the device is located on the end of the casing string it is generally referred to as a float shoe. If the device is located between adjacent joints of casing it is generally referred to as a float collar. During cementing of the casing, the check valve permits cement to flow downward through the casing and out into the annulus, but prevents back flow of cement from the annulus into the casing.
During lowering of the casing into the borehole, it is frequently necessary to open the check valve in order to allow fluid to flow upwardly therethrough. The need for opening the check valve during lowering of the casing into the borehole is caused by the presence of liquid-phase fluids in the borehole which exert an upward buoyancy force on the casing that is sufficient to float the casing in the borehole. Such liquid-phase fluids may include drilling mud and/or other wellbore fluids which are typically present in a borehole drilled using liquid-based drilling fluids.
In an air-drilled wellbore, however, the borehole is typically devoid of liquid-phase fluids which would be sufficient to float the casing. Rather, an air-drilled borehole typically contains primarily gas-phase fluids. Thus, when casing equipped with a check valve is lowered into an air-drilled borehole, it is not necessary to open the check valve and permit upward fluid flow into the casing in order prevent floating of the casing. In fact, in a air-drilled borehole it is undesirable to allow such upward fluid flow through the casing because the upward flow of gas-phase fluids through the casing may present a fire hazard at the top of the casing.
One problem encountered when cementing casing in an air-drilled wellbore is that the cement charged to the top of the casing free-falls downward through the gas-phase fluids in the casing. Because these gas-phase fluids provide only minimal resistance to the downward flow of the cement through the casing, the velocity of the cement falling through the casing can reach excessively high levels. When the high velocity cement reaches the bottom of the casing, it can cause large pressure surges which are transferred to the rock matrix. Pressure surge is undesirable because it can cause fracturing of the subterranean formation.
In accordance with an embodiment of the present invention, a wellbore cementing method is provided. The cementing method comprises the steps of: (a) lowering a casing into a borehole which contains fluids that are insufficient to float the casing; (b) charging cement to an upper end of the casing; and (c) restricting the downward flow of the cement through the casing with a cement choke.
In accordance with another embodiment of the present invention, a wellbore cementing method is provided. The wellbore cementing method comprises the steps of: (a) coupling a choke element to a float collar; (b) coupling the float collar between two adjacent joints of casing; (c) lowering the casing and the float collar into a borehole; (d) at least substantially blocking upper fluid flow through the float collar; (e) charging cement to the upper end of the casing so that the cement falls downward towards the float collar; and (f) contacting the cement with the choke element so that the velocity of the cement exiting the float collar is less than it would have been had step (a) not been performed.
In accordance with a further embodiment of the present invention a downhole choke couplable between two adjacent joints of wellbore casing is provided. The downhole choke comprises a tubular body, a seat, a choke element, and a check valve. The tubular body defines a fluid passageway. The seat is coupled to the tubular body and defines a seat orifice. The seat orifice is in fluid communication with the fluid passageway. The choke element is coupled to the seat and defines a choke orifice. The choke element is operable to at least partially inhibit fluid flow through the seat orifice in a first flow direction. The check valve is coupled to the seat and operable to at least substantially block fluid flow through the seat orifice in a second flow direction which is generally opposite the first flow direction.
In accordance with a still further embodiment of the present invention, a wellbore which has been readied for cementing is provided. The wellbore comprises a generally downwardly extending borehole, a casing string, and a cement choke. The casing string presents upper and lower ends and defines a fluid passageway therebetween. The casing string is disposed in the borehole and is at least substantially fixed relative to the borehole. The cement choke is coupled to the casing string below the upper end of the casing. The cement choke presents a flow restricting surface operable to at least partially inhibit the downward flow of cement through the fluid passageway and dampening pressure surges. The fluid passageway above the cement choke primarily contains gas-phase fluids.
In accordance with another embodiment of the present invention a method of making a downhole cement choke is provided. The downhole cement choke is made by modifying a conventional float collar which includes a seat presenting a seat opening and a check valve coupled to the seat and operable to provide one-way flow through the seat orifice. The seat defines a surface into which a conventional auto-fill valve can be mounted. The method of making the downhole cement choke comprises the steps of: (a) forming a choke element which defines a choke orifice having a flow area which is less than the flow area of the seat orifice; and (b) placing the choke element in registry with the surface which could hold the conventional auto-fill sleeve so that the choke element is spaced from the check valve.
The present invention provides a system for inhibiting the fracturing of subterranean formations when cementing casing in a wellbore. Other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures.